(Sources: CIA country guide, World Bank, the OEC, Global Firepower. Figures are for latest available year)

Bahrain, which is ruled by a Sunni monarchy but has a majority Shia population, on Monday gave Iranian diplomats 48 hours to leave the country.

It accused Iran of "increasing, flagrant and dangerous meddling" in the internal affairs of Gulf and Arab states.

Image copyrightAPImage caption
Protests have taken place in Bahrain against Sheikh Nimr al-Nimr's execution

It said the attack on the Saudi embassy was part of a "very dangerous pattern of sectarian policies that should be confronted... to preserve security and stability in the entire region".

Bahrain, which hosts the US Navy's 5th Fleet, has frequently accused Iran of fomenting unrest in the country since 2011 - a charge Tehran denies.

After demonstrations over the weekend, further protests were held by the Shia Muslim community in Bahrain on Monday.

A Sudan foreign ministry statement read: "In response to the barbaric attacks on the Saudi Arabian embassy in Tehran and its consulate in Mashhad... Sudan announces the immediate severing of ties with the Islamic Republic of Iran."

Sunni-Shia divide

The split arises from a dispute soon after the death of the Prophet Muhammad in 632 over who should lead the Muslim community

Sunnis are estimated to make up between 85% and 90% of Muslims

Though the two branches have co-existed for centuries and share many fundamental beliefs and practices, differences lie in the fields of doctrine, ritual, law, theology and religious organisation

Saudi Arabia announced late on Sunday that it was severing diplomatic relations with Iran after demonstrators stormed the Saudi embassy in Tehran. Protesters set the building on fire before being driven away by police.

Iran's foreign ministry on Monday accused the Saudis of "continuing the policy of increasing tension and clashes in the region".

Ministry spokesman Hossein Jaber Ansari said: "Saudi Arabia sees not only its interests but also its existence in pursuing crises and confrontations and attempts to resolve its internal problems by exporting them to the outside."

He defended Iran's response to the embassy attack, saying it had "acted in accordance with its obligations to control the broad wave of popular emotion". Fifty arrests were made.

Analysis: Jonathan Marcus, BBC diplomatic correspondent

One by one, Saudi Arabia's friends in the region are backing the kingdom in its growing dispute with Iran.

Ties between Iran and Bahrain have long been difficult; influenced by many of the same factors that have poisoned Saudi-Iranian relations.

Bahrain claims that Iran is behind much of the recent turmoil afflicting that country. The United Arab Emirates has downgraded relations, but will continue longstanding trade links with Iran.

The demise of Saddam Hussein in Iraq - who always served as an Arab counter-weight to Tehran - has resulted in a growing regional role for Iran, which the Gulf Arabs see as a serious threat.

The US-Iran nuclear deal has also increased sensitivities in the region as has the continuing struggle in Syria, in which the Gulf Arabs and Tehran are on very different sides.

In other developments:

A leaked memo by the commander of the Saudi interior ministry's Special Emergency Force, seen by the BBC, appears to show counter-terrorism units across the country were ordered to cancel holidays and operate at "100% capacity" from Friday morning - the day before the execution - as "maximum precaution"

US Secretary of State John Kerry and UN Secretary General Ban Ki-moon speak to Iranian and Saudi officials - Mr Ban urges both parties to "avoid any actions that could further exacerbate the situation", a UN spokesman says